The role of something close to random chance in all this is striking. Which worldlet will be shattered or ejected, and which will safely grow to planethood, is not obvious. There are so many objects in so complicated a set of mutual interactions that it is very hard to tell—just by looking at the initial configuration of gas and dust, or even after the planets have mainly formed—what the final distribution of worlds will be. Perhaps some other, sufficiently advanced observer could figure it out and predict its future—or even set it all in motion so that, billions of years later, through some intricate and subtle sequence of processes, a desired outcome will slowly emerge. But that is not yet for humans.
You started with a chaotic, irregular cloud of gas and dust, tumbling and contracting in the interstellar night. You ended with an elegant, jewel-like solar system, brightly illuminated, the individual planets neatly spaced out one from another, everything running like clockwork. The planets are nicely separated, you realize, because those that aren’t are gone.
——
It’s easy to see why some of those early physicists who first penetrated the reality of the nonintersecting, coplanar orbits of the planets thought that the hand of a Creator was discernible. They were unable to conceive of any alternative hypothesis that could account for such magnificent precision and order. But in the light of modern understanding, there is no sign of divine guidance here, or at least nothing beyond physics and chemistry. Instead we see evidence of a time of remorseless and sustained violence, when vastly more worlds were destroyed than preserved. Today we understand something of how the exquisite precision that the Solar System now exhibits was extracted from the disorder of an evolving interstellar cloud by laws of Nature that we are able to grasp—motion, and gravitation, and fluid dynamics, and physical chemistry. The continued operation of a mindless selective process can convert chaos into order.
Our Earth was born in such circumstances about 4.5 or 4.6 billion years ago, a little world of rock and metal, third from the Sun. But we musn’t think of it as placidly emerging into sunlight from its catastrophic origins. There was no moment in which collisions of small worlds with the Earth ceased entirely. Even today objects from space run into the Earth or the Earth overtakes them. Our planet displays unmistakable impact scars from recent collisions with asteroids and comets. But the Earth has machinery that fills in or covers over these blemishes—running water, lava flows, mountain building, plate tectonics. The very ancient craters have vanished. The Moon, though, wears no makeup. When we look there, or to the Southern Highlands of Mars, or to the moons of the outer planets, we find a myriad of impact craters, piled one on top of the other, the record of catastrophes of ages past. Since we humans have returned pieces of the Moon to the Earth and determined their antiquity, it is now possible to reconstruct the chronology of cratering and glimpse the collisional drama that once sculpted the Solar System. Not just occasional small impacts, but massive, stupefying, apocalyptic collisions is the inescapable conclusion from the record preserved on the surfaces of nearby worlds.
By now, in the Sun’s middle age, this part of the Solar System has been swept free of almost all the rogue worldlets. There is a handful of small asteroids that come near the Earth, but the chance that any of the bigger ones will hit our planet soon is small. A few comets visit our part of the Solar System from their distant homeland. Out there, they are occasionally jostled by a passing star or a nearby, massive interstellar cloud—and a shower of icy worldlets comes careening into the inner Solar System. These days, though, big comets hit the Earth very rarely.
Shortly, we will sharpen our focus to one world only, the Earth. We will examine the evolution of its atmosphere, surface, and interior, and the steps that led to life and animals and us. Our focus will then progressively narrow, and it will be easy to think of us as isolated from the Cosmos, a self-sufficient world minding its own business. In fact, the history and fate of our planet and the beings upon it have been profoundly, crucially influenced, through the whole history of the Earth and not just in the time of its origins, by what’s out there. Our oceans, our climate, the building blocks of life, biological mutation, massive extinctions of species, the pace and timing of the evolution of life, all cannot be understood if we imagine the Earth hermetically sealed from the rest of the Universe, with only a little sunlight trickling in from the outside.
The matter that makes up our world came together in the skies. Enormous quantities of organic matter fell to Earth, or were generated by sunlight, setting the stage for the origin of life. Once begun, life mutated and adapted to a changing environment, partially driven by radiation and collisions from outside. Today, nearly all life on Earth runs off energy harvested from the nearest star. Out there and down here are not separate compartments. Indeed, every atom that is down here was once out there.5
Not all of our ancestors made the same sharp distinction we do between the Earth and the sky. Some recognized the connection. The grandparents of the Olympian gods and therefore the ancestors of humans were, in the myths of the ancient Greeks, Uranus,6 god of the sky, and his wife Gaia, goddess of the Earth. Ancient Mesopotamian religions had the same idea. In dynastic Egypt the gender roles were reversed: Nut was goddess of the sky, and Geb god of Earth. The chief gods of the Konyak Nagas on the Himalayan frontier of India today are called Gawang, “Earth-Sky,” and Zangban, “Sky-Earth.” The Quiché Maya (of what is now Mexico and Guatemala) called the Universe cahuleu, literally “Sky-Earth.”
That’s where we live. That’s where we come from. The sky and the Earth are one.
Chapter 2
SNOWFLAKES FALLEN ON THE HEARTH
There is not yet one person, one animal, bird, fish, crab, tree, rock, hollow, canyon, meadow, forest. Only the sky alone is there …
Popol Vuh: The Mayan Book of the Dawn of Life1
Before the High and Far-Off Times, O my Best Beloved, came the Time of the Very Beginnings; and that was in the days when the Eldest Magician was getting Things ready. First he got the Earth ready; then he got the Sea ready; and then he told all the Animals that they could come out and play.
RUDYARD KIPLING
“The Crab That Played with the Sea”2
If you could drive an automobile straight down, in an hour or two you would find yourself deep inside the upper mantle of the Earth, far beneath the pediments of the continents, approaching an infernal region where the rock becomes a viscous liquid, mobile and red-hot. And if you could drive for an hour straight up, you would find yourself in the near-vacuum of interplanetary space.3 Beneath you—blue, white, breathtakingly vast, and brimming over with life—would stretch the lovely planet on which our species and so many others have grown up. We inhabit a shallow zone of environmental clemency. Compared to the size of the Earth, it is thinner than the coat of shellac on a large schoolroom globe. But earlier, long ago, even this narrow habitable boundary between hell and heaven was unready to receive life.